This invention relates to a three-step method of making a gas diffusion layer for an electrochemical cell by combining carbon particles and surfactant in a typically aqueous vehicle, typically by high shear mixing, adding fluoropolymer by low shear mixing, and applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
U.S. Pat. No. 6,127,059 describes a method of coating a gas diffusion layer for use in an electrochemical cell with a composition made by simultaneously mixing fluororesin and carbon black in water.
Briefly, the present invention provides a method of making a gas diffusion layer for an electrochemical cell comprising the steps of: a) combining a vehicle with carbon particles and one or more surfactants to make a preliminary composition, typically by high shear mixing; b) adding one or more highly fluorinated polymers to said preliminary composition by low shear mixing to make a coating composition; and c) applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
What has not been described in the art, and is provided by the present invention, is a three-step method of mixing and coating a carbon/fluororesin composition which provides the high shear treatment of the carbon particles but avoids high shear treatment of the fluororesin.
In this application:
xe2x80x9cvehiclexe2x80x9d means a fluid which carries the particulate in a dispersion, which typically includes water or an alcohol;
xe2x80x9chighly fluorinatedxe2x80x9d means containing fluorine in an amount of 40 wt % or more, but typically 50 wt % or more, and more typically 60 wt % or more;
xe2x80x9chigh shear mixingxe2x80x9d means a mixing process wherein the fluid to be mixed encounters zones of shear having a shear rate greater than 200 secxe2x88x921, and more typically greater than 1,000 secxe2x88x921, typified by mixing with a high speed disk disperser or Cowles blade at sufficient rpms;
xe2x80x9cultra high shear mixingxe2x80x9d means a mixing process wherein the fluid to be mixed encounters zones of shear having a shear rate greater than 10,000 secxe2x88x921, and more typically greater than 20,000 secxe2x88x921, typified by bead milling or sand milling at sufficient rpms;
xe2x80x9clow shear mixingxe2x80x9d means a mixing process wherein the fluid to be mixed does not substantially encounter zones of shear having a shear rate greater than 200 secxe2x88x921, more typically not greater than 100 secxe2x88x921, more typically not greater than 50 secxe2x88x921, and more typically not greater than 10 secxe2x88x921, typified by paddle mixing, hand stirring, or low-rpm mixing with a high speed disk disperser;
xe2x80x9clow shear coatingxe2x80x9d means a coating process wherein the fluid to be coated does not substantially encounter zones of shear having a shear rate greater than 2000 secxe2x88x921, more typically not greater than 1000 secxe2x88x921, more typically not greater than 500 secxe2x88x921, and more typically not greater than 100 secxe2x88x921, typified by three-roll coating;
xe2x80x9ccarbon bleed-throughxe2x80x9d refers to the presence of carbon particles on an uncoated side of an electrically conductive porous substrate which have migrated through the substrate from a coated side, typically in an amount sufficient to be visible to the naked eye or more; and
xe2x80x9csubstitutedxe2x80x9d means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide a method of making a coating composition for coating a gas diffusion layer for use in an electrochemical cell which provides both highly dispersed carbon and highly dispersed fluororesin.
The present invention provides a method of making a gas diffusion layer for an electrochemical cell comprising the steps of: a) combining a vehicle with carbon particles and one or more surfactants to make a preliminary composition, typically by high shear mixing; b) adding one or more highly fluorinated polymers to said preliminary composition by low shear mixing to make a coating composition; and c) applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
Fuel cells are electrochemical cells which produce usable electricity by the catalyzed combination of a fuel such as hydrogen and an oxidant such as oxygen. Typical fuel cells contain layers known as gas diffusion layers or diffuser/current collector layers adjacent to catalytically reactive sites. These layers must be electrically conductive yet must be able to allow the passage of reactant and product fluids. Typical gas diffusion layers are coated with a layer of carbon particles and fluoropolymers on the surface adjacent to the catalyst. This invention concerns improvements in coating the carbon/fluoropolymer layer. Specifically, the method according to the present invention provides for high shear treatment of the carbon particles, resulting in increased wetting-out and dispersion, but avoids high shear treatment of the fluororesin which can cause agglomeration.
The coating composition may employ any suitable aqueous vehicle. The vehicle comprises water and may additionally comprise alcohols, and more typically comprises only water or alcohols. Most typically the vehicle comprises water alone.
The coating composition may comprise any suitable surfactant or dispersant, including amine oxide surfactants described in co-pending patent application Ser. No. 10/028,173, filed on even date herewith and incorporated herein by reference. Suitable amine oxides may belong to formula II: R3Nxe2x86x92O, where each R is independently selected from alkyl groups containing 1-20 carbons, which optionally include ether and alcohol groups, and which may be additionally substituted. Typical amine oxide surfactants according to the cited disclosure are alkyl dimethylamine oxides according to formula (I): 
wherein n is 9 to 19 or more typically 11 to 15. Most typically, n is 11 or 13. The amine oxide according to formula (I) is optionally substituted. Suitable amine oxide surfactants may include those available under the trade names Genaminox(copyright), Admox(copyright), Ammonyx(copyright), and Ninox(copyright).
Other suitable surfactants may include alcohol alkoxylates such as Triton(trademark) X100.
The coating composition typically comprises 0.1-15% surfactant by weight, more typically 0.1-10% by weight, and most typically 1-5% by weight.
Any suitable carbon particles may be used. It will be understood that the term xe2x80x9ccarbon particlesxe2x80x9d as used herein can refer to primary particles, typically having a average size of 1-100 nm, primary aggregates of primary particles, typically having an average size of 0.01-1 microns, secondary aggregates of primary aggregates, typically having an average size of 0.1-10 microns, and agglomerates of aggregates, typically having an average size of greater than 10 micron. Most typically, the term xe2x80x9ccarbon particlesxe2x80x9d refers to primary particles or primary aggregates. Typically a carbon black is used, such as Vulcan XC-72 (Cabot Corp., Special Blacks Division, Billerica, Mass.), Shawinigan Black, grade C55, (Chevron Phillips Chemical Company, LP, Acetylene Black Unit, Baytown, Tex.) or Ketjenblack EC300J (Akzo Nobel Chemicals Inc., Chicago, Ill.). The aqueous coating composition typically comprises 1-50% carbon particles by weight, more typically 1-20% by weight, and most typically 5-15% by weight. Typically, the aqueous coating composition comprises lower weight percent content of carbon particles where smaller particles are used.
The carbon particles are typically suspended in the vehicle by high shear mixing to form a preliminary composition. High shear mixing advantageously provides improved wetting-out of carbon particles with the vehicle as well as improved dispersion and de-agglomeration. In addition, the preliminary composition may be degassed or defoamed by any suitable method, including standing. The preliminary composition may be further mixed by ultra high shear mixing, typically after degassing or defoaming.
A thickening agent may be added to the preliminary composition. Any suitable thickening agent may be used, including polyacrylates such as Carbopol(copyright) EZ-2 (B.F. Goodrich Specialty Chemicals, Cleveland, Ohio).
A defoaming agent may be added to the preliminary composition. Any suitable defoaming agent may be used, such as Mazu(copyright) DF 210 SX (BASF Corp., Mount Olive, N.J.).
Without wishing to be bound by theory, it is believed that the advantage of the method according tot he present invention derives from the fact that high shear mixing or ultra high shear mixing, which provides de-agglomeration of particulate carbon, can cause agglomeration of fluoropolymer in dispersion. The present method provides two-step mixing and low shear treatment of fluororesin-containing compositions and thus prevents or reverses agglomeration of both carbon and fluoropolymer particles.
Any suitable highly fluorinated polymers may be used. The highly fluorinated polymer is typically a perfluorinated polymer, such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkyl acrylates, hexafluoropropylene copolymers, tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymers, and the like. The aqueous coating composition typically comprises 0.1-15% highly fluorinated polymers by weight, more typically 0.1-10% by weight, and most typically 1-5% by weight. The highly fluorinated polymer is typically provided as an aqueous or alcoholic dispersion, most typically aqueous, but may also be provided as a powder.
Any suitable low shear method can be used to mix the fluororesin and the preliminary composition to form the coating composition, including paddle mixing and the like. Typically the low shear mixing process exposes the coating compositions to shear rates of no greater than 100 secxe2x88x921, more typically not greater than 50 secxe2x88x921, and more typically not greater than 10 secxe2x88x921.
The coating composition can be coated onto any suitable electrically conductive porous substrate. Typically, the electrically conductive porous substrate is a carbon fiber construction. Typically carbon fiber constructions are selected from woven and non-woven carbon fiber constructions. Carbon fiber constructions which may be useful in the practice of the present invention may include: Toray(copyright) Carbon Paper, SpectraCarb(trademark) Carbon Paper, AFN(copyright) non-woven carbon cloth, Zoltek Panex(copyright) Carbon Cloth, and the like. The porous substrate may be treated prior to coating. Typical treatments include those that increase or impart hydrophobic properties, such as treatment with fluoropolymers such as PTFE. Other typical treatments may increase or impart hydrophilic properties.
Any suitable method of coating may be used, however typically a low shear coating method is used. Typically the low shear mixing process exposes the coating compositions to shear rates of no greater than 100 secxe2x88x921, more typically not greater than 50 secxe2x88x921, and more typically not greater than 10 secxe2x88x921. Typical methods include both hand and machine methods, including hand brushing, notch bar coating, fluid bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife coating, and three-roll coating. Most typically three-roll coating is used. Advantageously, coating is accomplished without carbon bleed-through from the coated side of the substrate to the uncoated side. Coating may be achieved in one pass or in multiple passes. Coating in multiple passes may be useful to increase coating weight without corresponding increases in mud cracking.
The coated substrate may then be heated to a temperature sufficient to remove the vehicle and surfactants. The coated substrate may be heated to a temperature sufficient to sinter the highly fluorinated polymers.
The resulting gas diffusion layer is typically incorporated into a membrane electrode assembly for use in an electrochemical cell such as a hydrogen fuel cell by any suitable method, many of which are known in the art.
This invention is useful in the manufacture of a gas diffusion layer for use in electrochemical cells such as hydrogen fuel cells.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.